In a wireless communication system, for example, a Global System for Mobile (GSM) Communication system using TDMA (Time Division Multiple Access) signaling, the signaling format includes a framed structure comprising a number of time slots. The time slots serve as channels over which mobile and base stations transmit or receive information. Each channel or time slot of a frame is assigned to a different user, with mobile-to-base station (uplink) transmissions carried on one frequency band and base-to-mobile station (downlink) transmissions carried on a separate frequency band.
Transmissions in each channel or time slot are specified to ramp up to a required power level and ramp down to a required power level in a predetermined amount of time within the time slot. GSM specifications require that the power at the start and end of a burst must be at a specified minimum level and that the transition from the minimum level to the final required level must be completed in a specified amount of time. The rates of ramping up and down are specified in order to reduce the generation of transient side bands and interference on adjacent channels.
Referring to FIG. 1, therein is illustrated a prior art power control loop, denoted generally as 10. A variable attenuator 12 is coupled to the input of an amplifier chain 14. Variable attenuator 12 and amplifier chain 14 are coupled between input 16 and output 18. A bias signal, V.sub.c, is applied to the variable attenuator 12 on line 20 to control the attenuation characteristics of the variable attenuator 12. Bias signal V.sub.c controls attenuation levels in variable attenuator 12, allowing the power control loop 10 to maintain required power levels when ramping up, ramping down, and during the burst in a time slot of a TDMA signal.
Because of the limited detection range of linear detector 24 at output 18, typically about 45 dB, the operation of power control loop 10 is divided into an open-loop mode and a closed-loop mode. Power control loop 10 may have a dynamic range of about 80 dB in open-loop mode and will run in open-loop mode until the rate of ramp-up has reached a predetermined level, referred to as the switching point. At the switching point, the output 18 can be coupled by line 22 through linear detector 24 through a feedback loop in order to implement the closed-loop mode of operation.
In closed-loop mode, reference signal source 26 supplies reference signal, V.sub.r, which is proportional to the required rate of ramping defined by the GSM specification. V.sub.r is compared in comparator 28 to the detected signal, V.sub.d, on line 30. The difference, an error signal, V.sub.e, is applied to integrator 34 at line 32. Integrator 34 integrates error signal, V.sub.e, and applies the result, bias signal, V.sub.c, to variable attenuator 12 at line 20 to stabilize power levels when ramping up, ramping down, and during the burst of the TDMA signal. Integrator 34 may comprise a operational amplifier 36 having a non-inverting input coupled to ground through a resistor R.sub.1, a inverting input coupled to comparator 28 through a resistor R.sub.2 and an output coupled to line 20. Integrator 34 may further comprise a capacitor C coupled between the inverting input and the output of operational amplifier 36. V.sub.c may be calculated as: ##EQU1##
Prior to switching from open-loop mode to closed-loop mode, amplifier chain 14 is allowed to ramp up without correction to the level of bias signal V.sub.c, while detected signal V.sub.d equals zero or a constant offset voltage. Temperature variations may affect variable attenuator 12 and produce deviations in the attenuation characteristics of variable attenuator 12. The deviations may result in an increase or decrease in the rate of ramping. The increase or decrease in the attenuation characteristics of variable attenuator 12 may increase or decrease the output power, producing unwanted RF spectrum.
FIG. 2A is a plot illustrating deviations in the rate of ramping in the power control loop of FIG. 1 that are generated by changes in the attenuation characteristics of variable attenuator 12. A switching point transient can occur at a switching point 36 when a power control loop switches from open loop to close loop mode. The switching point transient may be caused by deviations in the rate of ramping as indicated by the short, dashed lines 36a and 36b. FIG. 2B is a plot illustrating a switching point transient 38a of the power control loop of FIG. 1 generated from an increase in the attenuation characteristics of variable attenuator 12. The increase in the attenuation characteristics results in a decrease in the power level at output 18. FIG. 2C is a plot illustrating a switching point transient 38b of the power control loop of FIG. 1 generated from a decrease in the attenuation characteristics of variable attenuator 12. The decrease in the attenuation characteristics results in an increase in the power level at output 18.
When power control loop 10 switches from open-loop mode to closed- loop mode with negative feedback, the deviation in the rate of ramping is automatically detected, and bias signal V.sub.c is adjusted to correct for the drift in the attenuation characteristics as a result of the offset signal generated. This immediate correction at the switching point creates transients 38a and 38b as illustrated in FIGS. 2B and 2C, which produces increases or decreases in voltage at output 18 and produces an RF spectrum resulting in adjacent channel interference. FIG. 2D illustrates the unwanted side band harmonics generated in the output power spectrum of power control loop 10 as a result of switching point transients.